Various heat treatment apparatuses are used in, for example, fabricating a semiconductor device to subject a semiconductor wafer, i.e., a workpiece, to an oxidation process, a diffusion process, a CVD (chemical vapor deposition) process and the like. A generally known heat treatment apparatus comprises a reaction tube (process tube) defining a processing chamber for containing a semiconductor wafer for processing, a resistance heating device disposed so as to surround the reaction tube, and a heat insulating structure disposed so as to surround the resistance heating device, and the resistance heating device is supported on the inner surface of the heat insulating structure by supporting members.
A resistance heating device, for example, for a heat treatment apparatus capable of carrying out a batch process employs a heating wire wound in a helical coil and disposed on the inner surface of a cylindrical heat insulating structure and is capable of heating the interior of a furnace at a high temperature of, for example, about 1200.degree. C. The heat insulating structure is, for example, is made by winding a heat insulating material, such as ceramic fibers, in a cylinder, and burning the cylinder. The heat insulating structure reduces the quantity of heat dissipated by radiation and conduction to enhance heating efficiency. The support members are made of, for example, a ceramic material and are disposed at predetermined intervals to support the heating wire so as to allow the thermal expansion and thermal contraction of the heating wire.
Incidentally, in the foregoing heat treatment apparatus, the heating wire is wound in a helical coil and is supported with a clearance between the heating wire and the heat insulating structure so as to be able to make thermal expansion and thermal contraction. However, the heating wire is subject to creep strain because the same is used at a high temperature, and the length of the heating wire increases with time. When energized for heating, the heating wire makes thermal expansion. The elongation of the heating wire appears at one end of the heating wire, the diameter of a coil of the heating wire at one end of the heating wire increases and the coil comes into contact with the heat insulating structure, a portion of the heating wire in contact with the heat insulating structure and portions of the same around the portion in contact with the heat insulating structure are subject to buckling deformation and are liable to be broken, which reduces the durability of the hating wire.
Particularly, in a vertical heat treatment apparatus, a heating wire is divided into a plurality of sections corresponding respectively to a plurality of zones, and the sections of the heating wire are fixed to a heat insulating structure by terminal strips. Therefore, elongations in the sections of the heating wire resulting from creep or thermal expansion are concentrated on the lowermost coils of the sections of the heating wire, and the lowermost coils of the sections of the heating wire come into contact with the heat insulating structure and become unable to absorb the elongations. Consequently, buckling deformation or the like of the heating wire occurs, the heating wire is liable to brake and the durability of the heating wire is deteriorated.